The Pentax astrotrace function is interesting and convenient when traveling in that the GPS unit is the size and weight of a matchbox.
I hae used it with my Pentax K5 APSc DSLR.
The tracking/exposure time is limited by the available movement of the sensor. Longer focal lengths have shorter max time than short focal lengths. Multiple subs are out because the sensor resets after each exposure.
The XY movement of the sensor can't exactly cancel the rotational sky motion. However it does a reasonable job. It does not replace or surpass the performance of a good equatorial tracking device or mount. I view it as a traveling convenience rather than something I use as a mainstay tracking technique.
You can see an example here, about 3/4 of the way down the page there is a 300s exposure with a 12mm lens.
http://www.joe-cali.com/eclipses/PAS...013/index.html
To activate it you have to do a precise calibration of the GPS. After turning it on you select precise calibration then you have to slowly rotate the camera 360 deg with GPS turned on in 3 Cartesian axes (X,Y,Z). This allows the GPS then sense pointing direction. The processor then computes how to minimize sky motion by 2D movement of the sensor. It's not as bad as guiding on an alt-az mount because the camera is pointing at the sky.
In the example, you'll see linear motion in top right and lower left corners which is an artifact of this strange tracking.
Cheers
Joe